Method of making an LCD or X-ray imaging device with first and second insulating layers

ABSTRACT

This invention is related to an active matrix liquid crystal display (AMLCD) or an X-ray imaging device having a high aperture ratio, and method of making same. The imager or display has an increased aperture ratio because electrodes are formed over dual insulating layers so as to overlap portions of the array address lines and/or TFTs. Both the manufacturability and capacitive crosstalk of the device are improved due to the use of a photo-imageable organic insulating layer between the pixel electrodes and the address lines. An intermediate inorganic insulating layer is provided between the photo-imageable organic insulating layer and the overlapped TFTs in order to prevent the organic insulating layer from directly contacting semiconductor material in the TFTs thereby reducing potential voltage swings.

RELATED PATENTS AND APPLICATIONS

This application is related to commonly owned U.S. Pat. No. 5,641,974,and commonly owned Ser. Nos. 08/470,271; 08/832,345; 08/671,376; and08/630,984, the disclosures of which are hereby incorporated herein byreference. Also, this application is related to a commonly ownedapplication filed simultaneously herewith, entitled X-RAY IMAGER OR LCDWITH BUS LINES OVERLAPPED BY PIXEL ELECTRODES AND DUAL INSULATING LAYERSTHEREBETWEEN.

This invention relates to an X-ray imager or a liquid crystal display(LCD) having an increased pixel aperture ratio, and method of makingsame. More particularly, this invention relates to such an imager orliquid crystal display including an array of TFTs wherein a pair ofinsulating layers having a plurality of contact vias or aperturesdisposed therein are located between (i) the TFTs and/or address linesand (ii) overlapping electrodes so that the electrodes are permitted tooverlap the row and column address lines and/or the TFTs withoutexposing the system to capacitive crosstalk.

BACKGROUND OF THE INVENTION

Electronic matrix arrays find considerable application in X-ray imagersand active matrix liquid crystal displays (AMLCDs). Such AMLCDs andX-ray imagers generally include X and Y (or row and column) addresslines which are horizontally and vertically spaced apart and cross at anangle to one another thereby forming a plurality of crossover points.Associated with each crossover point is an element (e.g. pixel) to beselectively addressed. These elements in many instances are liquidcrystal display pixels or alternatively the memory cells of anelectronically adjustable memory array or X-ray imaging array.

Typically, a switching or isolation device such as a diode or thin filmtransistor (TFT) is associated with each array element or pixel. Theisolation devices permit the individual pixels to be selectivelyaddressed by the application of suitable potentials between respectivepairs of the X and Y address lines. Thus, the TFTs act as switchingelements for energizing or otherwise addressing corresponding pixelelectrodes.

Amorphous silicon (a-Si) TFTs have found wide usage for isolationdevices in liquid crystal display (LCD) arrays and X-ray imagers.Structurally, TFTs generally include substantially co-planar source anddrain electrodes, a thin film semiconductor material (e.g. a-Si)disposed between the source and drain electrodes, and a gate electrodein proximity to the semiconductor but electrically insulated therefromby a gate insulator. In LCDs, current flow through a TFT between thesource and drain is controlled by the application of voltage to the gateelectrode. The voltage to the gate electrode produces an electric fieldwhich accumulates a charged region near the semiconductor-gate insulatorinterface. This charged region forms a current conducting channel in thesemiconductor through which current is conducted. Thus, by controllingthe voltage to the gate and drain electrodes, pixels may be switched onand off in a known manner.

Herein, “drain” electrodes are those which are in communication with adrain address line and “source” electrodes are those that are incommunication with the pixel electrodes through vias in the insulatinglayers.

It is old and well-known to make TFT arrays wherein address lines andoverlapping pixel electrodes are insulated from one another by aninsulating layer. For example, see U.S. Pat. Nos. 5,641,974; 5,055,899;5,182,620; 5,414,547; 5,426,523; 5,446,562; 5,453,857; and 5,457,553,the disclosures of which are incorporated herein by reference.

Unfortunately, when certain prior art pixel electrodes are arranged soas to overlap the address lines (e.g. U.S. Pat. No. 5,003,356), anundesirably high parasitic capacitance results in the overlap areasbetween the pixel electrodes and the address lines. In overlap areas,the pixel electrode forms a capacitor in combination with the overlappedaddress lines. The resulting parasitic capacitance C_(PL) between thepixel electrode and the address line in the area of overlap is definedas follows:

C _(PL)=(∈·∈₀ ·A)÷d

where “∈” is the dielectric constant of the insulating layer, “∈₀” is aconstant value of about 8.85×10⁻¹⁴ F/cm, “A” is the area of theresulting capacitor in the area of overlap, and “d” is the thickness ofthe insulating layer in the area of overlap.

When a thin profile silicon nitride insulator is used as in U.S. Pat.No. 5,003,356, the resulting parasitic capacitance created by theoverlap is undesirably high thereby resulting in capacitive crosstalk inthe LCD. The dielectric constant of silicon nitride is well over 5.0(typically from about 6.4 to 7.2). Such crosstalk results when thesignal voltage intended to be on a particular pixel is not there. Thus,when C_(PL) is too high, the voltage on the pixel is either higher orlower than intended depending upon how much voltage the other pixels onthe signal address line receive. In other words, the pixel is no longersatisfactorily isolated when C_(PL) is too high. In view of the above,there exists a need in the art for an LCD (and/or X-ray imager) havingboth an increased aperture ratio as well as reduced capacitive crosstalkin overlap areas so as to simultaneously and properly isolate each pixeland increase the pixel opening sizes.

Further with respect to the '356 patent, for example, its disclosuredoes not appreciate the importance of the dielectric constant ∈ of theinsulating layer. While referencing numerous materials including siliconnitride and SiO₂, which may be used for the insulating layer, the '356patent does not discuss either the dielectric constant values of thesematerials or their importance in helping reduce C_(PL) in overlap areas.When ∈ of the insulating layer is too high, capacitive crosstalkresults.

Recently, organic polymer films have been applied to TFT-LCDs as aninsulating layer between address lines and pixel electrodes in highaperture applications. For example, see commonly owned U.S. Pat. No.5,641,974. There has been increasing concern as to how such polymersaffect back-channel-etch TFT performance and reliability. It has beenfound that both threshold voltage and sub-threshold swing can bedegraded by acrylic or black resin insulating layers, as compared tosilicon nitride, for example. In addition, a high off-current shoulderin sub-threshold regions has been found after negative gate voltagestress on acrylic passivated TFTs. The mechanism behind such degradationis believed to be that fixed charge and defect states are created at theinterface between the organic insulating layer and the a-Si TFT layer.

As the performance of LCDs and X-ray imagers is dependent upon TFTcharacteristics in both the off-state and sub-threshold regions, thebelow-referenced invention is an improvement over the disclosure of thecommonly owned '974 patent in an effort to improve TFT characteristicsand performance.

It is apparent from the above that there exists a need in the art for animproved TFT array and/or resulting LCD (or X-ray imager) having anincreased pixel aperture ratio, good TFT performance in all regions, andlittle capacitive crosstalk. Such an LCD or X-ray imager should be madewith as few steps as possible.

It is a purpose of this invention to fulfill the above-described needsin the art, as well as other needs which will become apparent to theskilled artisan from the following detailed description of thisinvention.

SUMMARY OF THE INVENTION

Generally speaking, this invention fulfills the above-described needs inthe art by providing an X-ray imager comprising:

a substrate;

an array of thin film transistors (TFTs) disposed on the substrate, thearray of TFTs including a plurality of address lines connected to theTFTs;

an array of substantially transparent electrodes disposed on the firstsubstrate, a plurality of the electrodes in the array of electrodesoverlapping at least one of the address lines;

an organic photo-imageable insulating layer having a dielectric constantless than about 4.0 disposed on the substrate between the address linesand the electrodes at least in areas of overlap and areas adjacentsource electrodes of the TFTs;

an intermediate protective insulating layer disposed between the TFTsand the photo-imageable insulating layer so as to prevent the organicphoto-imageable insulating layer from directly contacting semiconductormaterial in channels of the TFTs thereby reducing potential shifts ofthreshold voltage and subthreshold swings in the TFTs; and

the photo-imageable insulating layer and the intermediate protectivelayer each having a plurality of contact vias defined therein, whereinthe electrodes are in electrical communication with corresponding TFTsource electrodes through the contact vias defined in the insulatinglayers.

In certain preferred embodiments, the organic photo-imageable insulatinglayer is a negative resist which includes BCB.

In certain preferred embodiments, the organic photo-imageable insulatinglayer has a dielectric constant of less than about 3.0.

In certain preferred embodiments, the organic photo-imageable insulatinglayer has a thickness of from about 0.9 μm to 2.75 μm, with thethickness in the areas of overlap, of course, being smaller than theabove-recited thickness.

In certain preferred embodiments, the intermediate insulating layer hasa thickness of from about 100 Å-1,000 Å.

This invention further fulfills the above-described needs in the art byproviding a liquid crystal display comprising:

a first substrate;

a liquid crystal layer;

an array of substantially transparent pixel electrodes on the firstsubstrate for permitting image data to be displayed to a viewer;

a plurality of gatelines and TFT gate electrodes on the first substrate;

a semiconductor layer patterned so as to remain in an array of TFTareas;

a source and drain electrode in each TFT area on the first substrate, aTFT channel being defined between a corresponding source and drainelectrodes of each TFT, thereby forming an array of TFTs on the firstsubstrate;

drain lines connected to the drain electrodes;

wherein a plurality of the pixel electrodes overlap at least one of agateline and a drain line thereby increasing the pixel aperture ratio ofthe display;

a substantially transparent photo-imageable insulating layer having adielectric constant less than about 4.0, the photo-imageable insulatinglayer being disposed on the first substrate between (i) the pixelelectrodes and (ii) the TFTs and the overlapped lines so as to insulatethe pixel electrodes from the overlapped lines and the TFTs; and

an intermediate insulating layer disposed between the photo-imageableinsulating layer and the TFTs so as to prevent the photo-imageableinsulating layer from directly contacting TFT portions which thephoto-imageable insulating layer overlap.

This invention further fulfills the above-described needs in the art byproviding a TFT array structure comprising:

an array of TFTs on a substrate, the TFTs being connected to acorresponding array of electrodes;

row and column address lines on the substrate for addressing the TFTs;

organic photo-imageable insulating means having a dielectric constantless than about 4.0 disposed between the electrodes and the addresslines so as to reduce crosstalk and permit the insulating means to bephotoimaged; and

an intermediate protective insulating layer disposed between thephoto-imageable insulating means and the TFTs so as to prevent theinsulating means from contacting semiconductor material of the TFTs.

This invention further fulfills the above-described needs in the art byproviding a method of making a TFT structure, including first and secondinsulating layers over the TFTs.

This invention still further fulfills the above-described needs in theart by providing a method of making an X-ray imager, the methodincluding the steps of providing a substrate, forming an array of TFTson the substrate, forming a first inorganic (e.g. silicon nitride)insulating layer over the TFTs, forming a second organic insulatinglayer over the first layer so that the first insulating layer preventsthe second insulating layer from directly contacting the semiconductormaterial of the TFTs, and forming an array of electrode members over thesecond insulating layer with each of the electrode members incommunication with one of the TFTs through a contact hole defined in thefirst and second insulating layers.

This invention further fulfills the above-described needs in the art byproviding a method of making a liquid crystal display, the methodcomprising the steps of providing a substrate, forming an array of TFTsor other switching elements on the substrate, forming first and secondinsulating layers over the TFTs or other switching elements, formingcontact holes in the first and second insulating layers proximate TFTs,and forming a plurality of pixel electrodes over the second insulatinglayer so that each of the pixel electrodes is in communication with oneof the TFTs through a corresponding one of the contact holes.

This invention will now be described with reference to certainembodiments thereof as illustrated in the following drawings.

IN THE DRAWINGS

FIG. 1 is a top view of an AMLCD according to this invention, thisfigure illustrating pixel electrodes overlapping surrounding row andcolumn address lines along their respective lengths throughout thedisplay's pixel area so as to increase the pixel aperture ratio of thedisplay.

FIG. 2 is a top view of the column (or drain) address lines andcorresponding drain electrodes of FIG. 1 (or of an X-ray imager), thisfigure also illustrating the TFT source electrodes disposed adjacent thedrain electrodes so as to define the TFT channels having length “L.”

FIG. 3 is a top view of the pixel electrodes of FIG. 1 (or of an X-rayimager) except for their extensions.

FIG. 4 is a side elevational cross-sectional view of the linear-shapedthin film transistors (TFTs) of FIGS. 1-2, this structure being utilizedin either an LCD or an X-ray imager according to this invention.

FIGS. 5-7 are side elevational cross-sectional views illustrating how aTFT in an array according to this invention is manufactured.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THIS INVENTION

Referring now more particularly to the accompanying drawings in whichlike reference numerals indicate like parts throughout the severalviews.

The structure and function of AMLCDs are disclosed in U.S. Pat. No.5,641,974, and the structure and function of X-ray imagers are disclosedin U.S. Pat. Nos. 5,525,527; 5,498,880; 5,396,072; and 5,619,033, thedisclosure of which are incorporated herein by reference. Familiaritywith such structures and functions is assumed herein.

FIG. 1 is a top view of four pixels in an array of an active matrixliquid crystal display (AMLCD) 2 according to an embodiment of thisinvention. This portion of the display includes substantiallytransparent pixel electrodes 3, drain address lines 5, gate addresslines 7, an array of four thin film transistors (TFTs) 9, and auxiliarystorage capacitors 11 associated with each pixel. Each storage capacitor11 is defined on one side by a gate line 7 and on the other side by anindependent storage capacitor electrode 12. Storage capacitor electrodes12 are formed along with drain electrodes 13. As shown, thelongitudinally extending edges of each pixel electrode 3 overlap drainlines 5 and gate lines 7 respectively along the edges thereof so as toincrease the pixel aperture ratio (or pixel opening size) of the LCD.

In the areas of overlap 18 between substantially transparent pixelelectrodes 3 and address or bus lines 5, 7, a pixel-line (PL) capacitoris defined by an electrode 3 on one side and the overlapped address lineon the other. The dielectric disposed between the electrodes of these PLcapacitors is insulation layer 33 (see FIGS. 4 and 7). The parasiticcapacitance C_(PL) of these capacitors is defined by the equation:$C_{PL} = \frac{\varepsilon \cdot \varepsilon_{0} \cdot A}{d}$

where “d” is the thickness of layer 33, ∈ is the dielectric constant oflayer 33, ∈₀ is the constant 8.85×10⁻¹⁴ F/cm (permitivity in vacuum),and “A” is the area of the PL capacitor in overlap areas 18. Thefringing capacitance may also be taken into consideration in a knownmanner. According to certain other embodiments, C_(PL) is less than orequal to about 0.01 pF for a display with a pixel pitch of about 150 μm.When the pixel pitch is smaller, C_(PL) should be scaled to a lowervalue as well because overlap areas 18 are smaller. Additionally, thepixel aperture ratio of an LCD decreases as the pixel pitch decreases asis known in the art. The pixel pitch of AMLCD 2 may be from about 40 to5,000 μm according to certain embodiments of this invention. The pixelpitch as known in the art is the distance between centers of adjacentpixels in the array.

According to alternative embodiments of FIG. 1, the TFTs may be swung90° so that the gates are formed by perpendicular extensions from thegate lines and the drains are formed by the drain lines themselves. Thismay be used in both LCD and X-ray imager embodiments of this invention.

FIG. 2 is a top view of drain address lines 5 of AMLCD 2 (or of an X-rayimager) showing how extensions of address lines 5 may form drainelectrodes 13 of TFTs 9 in the FIG. 1 illustrated embodiment. Each TFT 9in the array includes source electrode 15, 31. drain electrode 13, 29,and gate electrode 17. Gate electrode 17 of each TFT 9 is formed by thecorresponding gate address line 7 in the FIG. 1 illustrated embodiment.According to other embodiments discussed above, the gate electrode 17may be formed by a branch extending substantially perpendicular to thegate line, while drain electrodes 13 are formed by the drain lines 5themselves.

FIG. 3 is a top view illustrating pixel electrodes 3 (absent theirextension portions 38) of AMLCD 2 or an X-ray imager arranged in arrayform.

FIG. 4 is a side elevational cross-sectional view of a single thin filmtransistor (TFT) 9 in the TFT array of either an X-ray imager or AMLCD2, with each TFT 9 in the array being substantially the same accordingto preferred embodiments. Each linear TFT 9 has a channel length “L”defined by the gap 27 between source electrode 15 and drain electrode13. Source electrode 15 is connected to pixel electrode 3 by way of viaor contact hole 35 so as to permit TFT 9 to act as a switching elementand selectively energize a corresponding pixel in AMLCD 2 in order toprovide image data to a viewer, or to address the memory cell of anX-ray imager. An array of these TFTs 9 is provided as illustrated inFIG. 1.

In X-ray imager embodiments, electrode 3 (e.g. of ITO) in FIG. 4overlaps the TFT channel 27, as shown in FIG. 4 while in certain LCDembodiments electrode 3 overlaps the source 15 but not channel 27. InX-ray imager embodiments, a separate collecting capacitor may beprovided as illustrated and discussed in U.S. Pat. Nos. 5,498,880 and5,396,072, or a separate photosensitive element as in U.S. Pat. No.5,619,033, the disclosures of which are hereby incorporated herein byreference. X-ray imagers according to this invention function asdiscussed in any of the aforesaid three U.S. patents.

Still referring to FIG. 4, each TFT 9 structure in both LCD and X-rayimager embodiments includes substantially transparent substrate 19 (e.g.made of glass), metal gate electrode 17, gate insulating layer or film21, semiconductor layer 23 (e.g. intrinsic amorphous silicon), dopedsemiconductor contact layer 25, drain electrode 13, source electrode 15,substantially transparent intermediate insulating layer 32,substantially transparent second insulation layer 33, and acorresponding substantially transparent electrode 3. Overlapping the TFTand/or address line(s). TFT channel 27 of length “L” is defined betweensource 15 and drain 13.

As shown in FIG. 4, drain electrode 13 is made up of a drain metal layer(e.g. Mo) which is deposited on substrate 19 over top of doped contactlayer 25. Contact film or layer 25 may be, for example, amorphoussilicon doped with an impurity such as phosphorous (i.e. n+a-Si) and issandwiched between semiconductor layer 23 and drain metal layer 13.Source electrode 15 includes doped semiconductor contact layer 25 andsource metal layer 15. Metal layers 13 and 15 may be of the same metaland deposited and patterned together according to certain embodiments ofthis invention. Alternatively, layer 13 may be deposited and patternedseparately from layer 15 so that drain metal layer is of one metal (e.g.Mo) and the source metal layer is of another (e.g. Cr).

Substantially transparent layers 32 and 33 make up the dual-layeredinsulator according to this invention, where layer 32 is provided toimprove TFT performance by preventing direct contact between organic oracrylic layer 33 and intrinsic a-Si layer 23. Thus, insulating layer 33with a low dielectric constant allows crosstalk to be reduced so as toimprove TFT and image performance while intermediate insulating layer 32(e.g. silicon nitride, silicon oxide, or SiO_(X)N_(Y)) eliminatespotential TFT off current and large subthreshold slope, and improves TFTthermal and electrical stability.

Substantially transparent insulating layer 33 having a dielectricconstant less than about 5.0 (preferably less than about 4.0 and mostpreferably less than about 3.0) is deposited as a sheet on substrate 19so as to cover inorganic intermediate insulating layer 32, TFTs 9, andaddress lines 5 and 7. Organic layer 33 is formed of a photo-imageablematerial such as Fuji Clear™ or a photo-imageable type of BCB.Substantially transparent layer 33 may also be formed of anon-photo-imageable type of BCB. Insulating layers 32 and 33 arecontinuous in the viewing area of the display or imager except for viasor contact holes 35 (and sometimes 36) formed therein to allowelectrodes 3 to contact corresponding TFT source electrodes 15 and thestorage capacitor electrodes respectively.

Layer 33 has a dielectric constant ∈ less than or equal to about 5.0according to certain embodiments of this invention, preferably less thanabout 4.0, and more preferably less than about 3.0. In certain preferredembodiments, layer 33 has a dielectric constant of about 2.7 and is madeof a photo-imageable type of Benzocyclobutene (BCB), an organic materialavailable from Dow Chemical, for the purpose of reducing capacitivecrosstalk (or capacitive coupling) between pixel electrodes 3 and theaddress lines in overlap areas 18. Layer 33 has a low dielectricconstant and/or a relatively large thickness for the specific purpose ofreducing C_(PL) in overlap areas 18.

Alternatively, layer 33 may be of a photo-imageable material known asFuji Clear™, which is an organic mixture including 2-Ethoxyethyl acetate(55-70%), methacrylate derivative copolymer (10-20%), and polyfunctionalacrylate (10-20%). Fuji Clear™ has a dielectric constant of about 3.5.Other materials which may be used as layer 33 includes PFCB availablefrom Dow, and known polyimides.

Inorganic layer 32 may be of any material which either eliminates orlessens the degree of TFT threshold voltage and sub-threshold swings, bypreventing direct contact between semiconductor material of the TFT andorganic or acrylic insulating layer 33.

Insulating layer 32 is formed on the TFT array over substantially theentire viewing area. Optionally, layer 32 may be formed only over theTFTs. Following the deposition of insulation layer 33 over top of layer32, TFTs 9, and address lines 5 and 7, vias 35 are formed in insulationlayer 33 by way of photo-imaging. Layer 33 acts as a negative resist sothat UV exposed areas remain on the substrate and areas of layer 33unexposed to UV during photo-imaging are removed during developing.Thereafter, using patterned layer 33 as an etch mask, vias 35 are etchedin intermediate layer 32 so as to expose the source electrodes 15. ForSiN_(X), SiO_(X), and/or SiO_(X)N_(Y) film, conventional reactive ionetching (RIE) utilizing F or Cl plasma, can be used. Following theforming of vias 35 in layers 32 and 33, substantially transparentelectrodes 3 (e.g. made of indium-tin-oxide or ITO) are deposited andpatterned over top of layers 32 and 33 on substrate 19 so that eachelectrode 3 contacts a corresponding source electrode 15 of acorresponding TFT 9 through a via 35 as illustrated in FIG. 4.

In certain embodiments, auxiliary vias 36 (see FIG. 1) are formed inlayers 32 and 33 at the same time as vias 35 so that electrodes 3 cancontact storage capacitor electrodes 12 via electrode extensions 38.Peripheral lead areas and seal areas are also removed by photo-imaging.

Insulating layers 32 and 33 are deposited on substrate 19 over theaddress lines, storage capacitors, and TFTs to a thickness “d” of atleast about 0.5 μm in overlap areas 18. In preferred embodiments, thetotal thickness “d” of the insulating layers 32 and 33 is from about 1to 2.8 μm, with layer 32 being from about 100-1,000Å thick (preferablyfrom about 500-1,000Å thick) and layer 33 being from about 0.90-2.75 μmthick (preferably from about 1.5-2.5 μm thick).

Another advantage of layer 33 is that liquid crystal layer disclinationsinduced at pixel electrode 3 edges by the topography of TFTs 9, storagecapacitors, and address lines are substantially eliminated byplanarization (i.e. few, if any, hills and valleys are present in thetop surface of layer 33). Thus, the thickness of the LC layer issubstantially maintained and display functionality is improved becauseelectrodes 3 are substantially flat because of the substantialplanarization of the surface of layer 33 adjacent the pixel electrodes3.

Because of the low dielectric constant ∈ and/or relatively highthickness “d” of layer 33, the capacitive crosstalk problems of theprior art resulting from overly high C_(PL) values are substantiallyreduced in areas 18 where pixel electrodes 3 overlap address lines 5and/or 7. Because layer 33 is disposed between pixel electrodes 3 andthe overlapped address lines, the capacitive crosstalk problems of theprior art are substantially reduced or eliminated and increased pixelopenings are achievable without sacrificing display performance (pixelisolation).

Pixel opening sizes or the pixel aperture ratio of AMLCD 2 (or of animager) is at least about 65% (preferably from about 68% to 80%)according to certain embodiments of this invention when the pixel pitchis about 150 μm. This will, of course, vary depending upon the pixelpitch of the X-ray imager or display (pitches of from about 40-500 μmmay be used). Electrodes 3 overlap address lines 5 and 7 along the edgesthereof as shown in FIG. 1 by an amount up to about 3 μm. In certainpreferred embodiments of this invention, the overlap 18 of electrodes 3over the edges of address lines 5 and 7 is designed to be from about 2to 3 μm, with the end result after overetching being at least about 0.5μm. According to certain other embodiments of this invention, the amountof overlap may be designed to be from about 2-3 μm, with the resultingpost-processing overlap being from about 0 to 2 μm. The overlap amountmay be adjusted in accordance with different LCD or imager applicationsand pitch sizes as will be appreciated by those of skill in the art.

In certain situations, after etching and processing, electrodes 3 maynot overlap the address lines at all according to certain embodiments ofthis invention, although some overlap 18 is preferred. When no overlapoccurs, the parasitic capacitance C_(PL) between the address lines andthe adjacent electrode 3 is still minimized or reduced due to insulatinglayer 33.

The TFT structures according to this invention have C_(PL) line-pixelcapacitance values, given the stated overlap and thickness parameters,as in U.S. Pat. No. 5,641,974. Thus, C_(PL) in areas of overlap ofaddress lines is preferably less than about 12.0 fF, and more preferablyless than about 9.0 fF.

Referring now to FIGS. 4-7, it will be described how an AMLCD 2 or anX-ray imager including the array of TFTs and corresponding address linesis made according to an embodiment of this invention. Firstly, referringto FIG. 5, substantially transparent substrate 19 is provided. Next, agate metal layer or sheet (which results in gate electrodes 17 and insome embodiments lines 7) is deposited on the top surface (surface to beclosest to the LC layer LCDs or the image to be received in imagers) ofsubstrate 19 to a thickness of from about 1,000-5,000 Å, most preferablyto a thickness of about 2,500 Å. The gate metal sheet is deposited byway of sputtering or vapor deposition. The gate metal may be of tantalum(Ta) according to certain embodiments of this invention. Insulatingsubstrate 19 may be of glass, quartz, sapphire, or the like.

The structure including substrate 19 and the deposited gate metal isthen patterned by photolithography to the desired gate electrode 17 andgate address line 7 configuration. The upper surface of the gate metalis exposed in a window where the photoresist has not been retained. Thegate metal (e.g. Ta) layer is then dry etched (preferably using reactiveion etching) in order to pattern the gate metal layer in accordance withthe retained photoresist pattern. To do this, the structure is mountedin a known reactive ion etching (RIE) apparatus which is then purged andevacuated in accordance with known RIE procedures and etchants. Thisetching of the gate metal layer is preferably carried out until the gatemetal is removed in center areas of the windows and is then permitted toproceed for an additional time (e.g. 20 to 40 seconds) of overetching toensure that the gate metal is entirely removed from within the windows.The result is gate address lines 7 and gate electrodes 17 being left onsubstrate 19.

After gate address lines 7 are deposited and patterned on top ofsubstrate 19 in the above-described manner, substantially transparentgate insulating or dielectric layer 21 is deposited over substantiallythe entire substrate 19, preferably by plasma enhanced chemical vapordeposition (CVD) or some other process known to produce a high integritydielectric. The resulting structure is shown in FIG. 5. Gate insulatinglayer 21 is preferably silicon nitride (e.g. Si₃N₄) but may also besilicon dioxide or other known dielectrics. Silicon Nitride has adielectric constant of about 6.4. Gate insulating layer 21 is depositedto a thickness of from about 2,000-3,000 Å (preferably either about2,000 Å or 3,000 Å) according to certain embodiments.

It is noted that after anodization (which is optional), gate Ta layer 17which was deposited as the gate electrode and gate line layer (whenoriginally about 2,500 Å thick) is about 1,800 Å thick and a newlycreated TaO layer is about 1,600 Å. Anodization takes place after thegate line patterning and before further processing. Thus, gateinsulating layer 21 over gate lines 7 and electrodes 17 is made up ofboth the anodization created TaO layer and the silicon nitride layer.Other metals from which gate electrode 17 and address line layer 7 maybe made include Cr, Al, titanium, tungsten, copper, and combinationsthereof.

Next, referring to FIG. 6, after gate insulating layer 21 has beendeposited (FIG. 5), semiconductor (e.g. intrinsic a-Si) layer 23 isdeposited on top of gate insulating layer 21 to a thickness of about2,000 Å. Semiconductor layer 23 may be from about 1,000 Å to 4,000 Åthick in certain embodiments of this invention. Then, doped (typicallyphosphorous doped, that is n+) amorphous silicon contact layer 25 isdeposited over intrinsic a-Si layer 23 in a known manner to a thicknessof, for example, about 500 Å. Doped contact layer 25 may be from about200 Å to 1,000 Å thick according to certain embodiments of thisinvention. The result is the FIG. 6 structure.

Gate insulating layer 21, semiconductor layer 23 and semiconductorcontact layer 25 may all be deposited on substrate 19 in the samedeposition chamber without breaking the vacuum according to certainembodiments of this invention. When this is done, the plasma dischargein the deposition chamber is stopped after the completion of thedeposition of a particular layer (e.g. insulating layer 21) until theproper gas composition for deposition of the next layer (e.g.semiconductor layer 23) is established. Subsequently, the plasmadischarge is re-established to deposit the next layer (e.g.semiconductor layer 23). Alternatively, layers 21, 23, and 25 may bedeposited in different chambers by any known method.

Following the formation of the FIG. 6 structure, the TFT islands orareas may be formed by way of etching of the semiconductor layers acrossthe array, for example, so that the TFT metal layers can be depositedthereon. Optionally, one of the TFT metal source/drain layers may bedeposited before forming the TFT island.

According to preferred embodiments, following the formation of the TFTisland from the FIG. 6 structure, a source-drain metal sheet or layer(which results in drain metal layer 13 and source metal layer 15) isdeposited on substrate 19 over top of semiconductor layer 23 and contactlayer 25. This source-drain metal layer may be chromium (Cr) ormolybdenum (Mo) according to certain embodiments of this invention. Whenchromium, the layer is deposited to a thickness of about 500-2,000 Å,preferably about 1,000 Å according to certain embodiments. Whenmolybdenum, the layer is deposited to a thickness of from about 2,000 to7,000 Å, preferably about 5,000 Å. The deposited source drain metallayer sheet is then patterned (masked and etched) to form the source 15,drain 13, and in some embodiments the storage capacitor electrodes.

Alternatively, a first metal layer may be deposited and patterned toform a drain electrode portion 13, and a second metal layer may bedeposited and patterned to form a source electrode portion 15. Thus, forexample, source metal layer 15 may be chromium (Cr) while drain metal 13and storage capacitor electrode layer is Mo according to certainembodiments of this invention. Other metals which may be used for thesource and drain metals include titanium, Al, tungsten, tantalum,copper, or the like.

After patterning of drain and source portions, contact layer 25 isetched in the TFT channel 27 area and inevitably a bit of semiconductorlayer 23 is etched along with it. The result is TFT 9 with channel 27 asshown in FIGS. 4 and 7.

After TFTs 9 are formed across the viewing area of the display orimager, substantially transparent dielectric intermediate protectivelayer 32 is deposited to a thickness of from about 100-1,000 Å(preferably from about 500 Å to 1,000 Å) on substrate 19 via CVD or thelike. This substantially transparent intermediate protective layer 32preferably includes or is of silicon nitride, but may be of other knownpassivation materials such as SiO₂, SiO_(X)N_(Y), or the like.Substantially transparent organic polymer insulating layer 33 is thendeposited onto substantially the entire substrate 19 over top ofintermediate layer 32 by way of spin-coating according to certainembodiments of this invention. Layer 33 may be of either photo-imageableBCB or acrylic Fuji Clear™ according to certain embodiments. As layer 33is much thicker than layer 32, Insulating layer 33 fills recessesgenerated upon formation of TFTs 9 and flattens the surface abovesubstrate 19 at least about 60% planarization according to certainembodiments.

Photo-imageable insulating layer 33 acts as a negative resist layeraccording to certain embodiments of this invention so that no additionalphotoresist is needed to form vias in layer 33. In order to form thevias, layer 33 is irradiated by ultraviolet (UV) rays (e.g. i rays of365 nm), with UV irradiated areas of layer 33 to remain and non-exposedor non-radiated areas of layer 33 to be removed in developing. A maskmay be used. Thus, the areas of the negative resist 33 corresponding tovias 35 (and in some embodiments 36) are not exposed to the UVradiation, while the rest of the layer 33 across the substrate isexposed to UV.

Referring to FIG. 7, following exposure of layer 33 (except in the viaor contact hole areas), layer 33 is developed by using a knowndeveloping solution at a known concentration. In the developing stage,the areas of layer 33 corresponding to the vias are removed (i.e.dissolved) so as to form the vias in the insulating layer 33. Afterdeveloping, the resist layer 33 is cured or subjected to postbaking(e.g. about 240 degrees C. for about one hour) to eliminate the solventso that the layer 33 with the vias therein is resinified. Thus, no dryor wet etching is needed to form the vias (e.g. 35) in layer 33.According to alternative embodiments, layer 33 may be a positive resistas opposed to a negative resist. The resulting structure is shown inFIG. 7.

After layer 33 is patterned (see FIG. 7), vias 35 are formed inintermediate layer 32 by RIE or the like, using the patterned layer 33as an etch mask. Conventional reactive plasma CHF₃/O₂; CF₄/O₂; SF₆/O₂;Cl₂; and/or NF₃, can be used to etch SiO_(X), SiN_(X), and/orSiO_(x)N_(y) films.

Vias or apertures 35 are thus formed in insulation layers 32 and 33 overtop of (or adjacent) each source metal electrode 15 so as to permitelectrodes 3 to electrically contact corresponding source electrodes 15through vias 35. Layers 32 and 33 remains across the rest of thesubstrate or array except for the storage capacitor vias (if needed) andcertain edge areas where contacts must be made or glueing done.

An alternative way to form layer 32 and/or layer 33 is to deposit layer32 first, and then pattern layer 32 with photoresist and etch layer 32.After layer 32 is finished, layer 33 is then deposited and patterned.This approach will add an additional photo step, but is feasible.

After the vias are formed in layers 32 and 33, a substantiallytransparent conducting layer (e.g. ITO) which results in electrodes 3 isdeposited and patterned (photomasked and etched) on substrate 19 overtop of layer 33. After patterning (e.g. mask and etching) of thissubstantially transparent conducting layer, electrodes 3 are left asshown in FIGS. 1 and 4. As a result of vias 35 formed in layer 33, eachelectrode 3 contacts a TFT source electrode 15 as shown in FIG. 4. Theresult is the active plate of either an LCD or an X-ray imager,including an array of TFTs. The electrode layer (when made of ITO) 3 isdeposited to a thickness of from about 500 to 3,000 Å (preferably about1,400 Å) according to certain embodiments of this invention. Other knownmaterials may be used as electrode layer 3. It is noted that in X-rayimager embodiments, electrodes 3 are preferably ITO, but need not betransparent.

Thus, with one extra deposition step and one extra RIE step, butpossibly no extra photo step, this invention allows fabrication of TFTswith higher performance and reliability.

After formation of the active plate, in LCD embodiments, liquid crystallayer 45 is disposed and sealed between the active plate and the passiveplate, the passive plate including substrate 51, polarizer 53, electrode49, and orientation film 47 as shown in the '974 patent.

Electrodes 3 are patterned to a size so that they overlap both drainaddress lines 5 and gate address lines 7 along the edges thereof so asto result in an increased aperture ratio for the imager or the AMLCD 2.Optionally, electrodes 3 need not overlap all address lines, but mayonly partially overlap certain address lines 5 or 7 in certainembodiments. Electrodes also overlap the TFT channels in X-ray imagerembodiments as shown in FIG. 4, but only partially overlap the TFTs anddo not overlap the TFT channels in certain LCD embodiments.

The crosstalk problems of the prior art are substantially eliminated dueto the presence of low dielectric layer 33 in overlap areas 18 betweenelectrodes 3 and the address lines, and TFT performance is improved bythe presence of intermediate layer 32.

Once given the above disclosure, many other features, modifications, andimprovements will become apparent to the skilled artisan. Such otherfeatures, modifications, and improvements are, therefore, considered tobe a part of this invention, the scope of which is to be determined bythe following claims.

We claim:
 1. A method of making an X-ray imaging device, the methodcomprising the steps of: providing a substrate; forming an array of thinfilm transistors (TFTs) on the substrate, the array of TFTs including aplurality of address lines connected to the TFTs; forming a firstinsulating layer over the TFTs on the substrate; forming an aphoto-imageable, organic polymer based second insulating layer includingacrylic over the first insulating layer on the substrate; forming aplurality of contact holes in the second insulating layer byphotoimaging and forming corresponding contact holes in the firstinsulating layer by etching using the second insulating layer as an etchmask; and forming an array of electrode members on the substrate overthe second insulating layer so that the electrode members in the arrayare in communication with corresponding TFTs through the contact holes,whereby the first insulating layer prevents the second insulating layerfrom directly contacting the TFTs.
 2. The method of claim 1, wherein theorganic polymer based second insulating layer is a negative resistlayer.
 3. The method of claim 1, wherein the organic polymer basedsecond insulating layer includes one of the 2-Ethoxyethyl acetate andBenzocyclobutene (BCB).
 4. The method of claim 1, wherein each of saidfirst and second insulating layers is substantially transparent tovisible wavelengths, and wherein the dielectric constant of the organicpolymer based second insulating layer is less than or equal to about4.0.
 5. The method of claim 4, wherein the dielectric constant of thesecond insulating layer is less than or equal to about 3.0.
 6. Themethod of claim 1, wherein the first insulating layer includes one ofsilicon nitride and silicon oxide.
 7. The method of claim 1, wherein theorganic polymer based second insulating layer has a thickness in certainareas of from about 0.9 to 2.75 μm, and said first insulating layer hasa thickness in certain areas from about 100 Å-1,000 Å.
 8. The method ofclaim 1, further comprising the step of patterning the electrode membersso that a plurality of the electrode members overlap at least a portionof one of the address lines so as to form a high aperture device, andwherein the address line—electrode capacitance C_(PL) in overlap areasis less than about 12.0 fF.
 9. The method of claim 8, wherein thecapacitance C_(PL) is less than about 9.0 fF.
 10. A method of making aliquid crystal display, the method comprising the steps of: providing afirst substrate; providing a liquid crystal layer; forming an array ofTFTs on the first substrate, each of the TFTs being in communicationwith address lines also on the first substrate; forming a firstpassivating insulating layer over the TFTs and address lines on thefirst substrate; forming an organic second insulating layer over thefirst passivating insulating layer, the second insulating layer beingphoto-imageable, including an acrylic, and having a dielectric constantvalue of less than or equal to about 4.0, said first and secondinsulating layers each being substantially transparent to visiblewavelengths of light; photoimaging an array of contact holes in thefirst insulating layer using the second insulating layer as an etch maskand etching corresponding contact holes in the second insulating layer;and forming an array of pixel electrodes over the second insulatinglayer so that each of the pixel electrodes is in communication with acorresponding TFT through a corresponding one of the contact holesdefined in the first and second insulating layers and the firstinsulating layer prevents the acrylic inclusive second insulating layerfrom directly contacting TFTs.
 11. The method of claim 10, wherein thesecond insulating layer is in direct contact with the first insulatinglayer, and the first insulating layer is in direct contact with at leasta portion of each TFT.
 12. The method of claim 10, wherein each of theTFTs includes a gate electrode, a drain electrode, and a sourceelectrode, the drain electrode of each TFT being in communication withthe drain address line and the gate electrode of each TFT being incommunication with a gate address line, and wherein each of the pixelelectrodes is in communication with one of the source electrodes of acorresponding TFT through a corresponding one of the contact holes. 13.The method of claim 12, wherein each of the TFTs includes asemiconductor layer located over the gate electrode, and wherein thesource and drain electrodes are located over the semiconductor layer ineach TFT.
 14. The method of claim 13, wherein the first insulating layerprevents the second insulating layer from directly contactingsemiconductor material in the TFTs so as to reduce potential shifts ofthreshold voltage and subthreshold swings in the TFTs.
 15. The method ofclaim 10, wherein the second insulating layer has a dielectric constantof less than or equal to about 3.0.
 16. The method of claim 10, whereinthe second insulating layer has a thickness in some areas of from about0.9 to 2.75 μm.
 17. The method of claim 16, wherein the first insulatinglayer is inorganic and has a thickness in some areas of from about 100Å-1,000 Å.
 18. The method of claim 10, wherein the display is a highaperture display in that each of the pixel electrodes overlaps at leastone of the address lines so as to provide an increased pixel apertureratio as compared to non-high aperture displays in which no such overlapis provided.
 19. A method of making a TFT structure, the methodcomprising the steps of: providing a substrate; forming an array ofsemiconductor based TFTs on the substrate, wherein each of the TFTs isin communication with at least one address line; forming a firstinsulating layer on the substrate over top of the TFTs and addresslines; forming an acrylic inclusive second organic insulating layer overtop of the first insulating layer on at least the TFTs and addresslines; providing the second insulating layer with a dielectric constantvalue of less than or equal to about 4.0; forming an array of vias orcontact holes in the second insulating layer by photoimaging and formingcorresponding vias or contact holes in the first insulating layer byetching using the second insulating layer as an etch mask; and formingan array of electrode members on the substrate so that each of theelectrode members is in communication with a corresponding one of theTFTs through a corresponding one of the contact holes.
 20. The method ofclaim 19, wherein each of the first and second insulating layers issubstantially transparent to visible wavelengths of light.